Field of the Invention
The present invention relates to a growth method for a microbe in a saccharified solution containing a saccharification product, and a bioethanol production method which forms ethanol by fermenting a saccharified solution with a microbe allowed to grow by the growth method.
Description of the Related Art
In recent years, there has been a demand for reduction in carbon dioxide emission, which is considered to be a cause of global warming, and use of biomass-derived diesel fuel or blended fuel of liquid hydrocarbon (e.g., gasoline) and biomass-derived ethanol as automobile fuel has been studied. Ethanol obtained by the fermentation of plant materials such as crops including sugarcane and corn can be used as the ethanol.
The amount of carbon dioxide emitted upon combustion of the ethanol obtained from such plant materials is equal to that absorbed by the raw material plant itself, because the plant itself has already absorbed carbon dioxide through photosynthesis. This means that the carbon dioxide emission is theoretically zero-sum, i.e., so-called carbon neutral effect can be obtained.
The crops such as sugarcane and corn are supposed to be food. In this respect, the problem is that the supply of these crops as food is decreased if they are consumed in large amounts as raw materials for ethanol.
Accordingly, techniques have been studied to produce ethanol using lignocellulosic biomass (hereinafter, also simply referred to as biomass), which is not used for food, instead of the crops (e.g., sugarcane and corn) as raw materials. The lignocellulosic biomass contains cellulose and hemicellulose (hereinafter, also collectively referred to as celluloses). These celluloses can be degraded with saccharifying enzymes into sugars, such as glucose or xylose, and the obtained sugars can be fermented to obtain ethanol.
Examples of the lignocellulosic biomass can include wood, rice straw, wheat straw, bagasse, bamboo, stalks, leaves, and cobs of corn, pulp, and wastes resulting therefrom, for example, waste paper.
The lignocellulosic biomass has hemicellulose and lignin as main constituents, in addition to cellulose. Since cellulose and hemicellulose are usually bound tightly to lignin, these celluloses are difficult to saccharify through direct reaction.
Thus, the lignocellulosic biomass-derived ethanol has heretofore been produced as follows: first, by the pretreatment of lignocellulosic biomass, lignin is dissociated from the lignocellulosic biomass or the lignocellulosic biomass is swollen to obtain a pretreatment product.
In the present specification, the dissociation refers to the cleavage of at least some bonds of sites at which lignin is bound to cellulose or hemicellulose in the lignocellulosic biomass. The swelling refers to the entrance of a liquid to form gaps in cellulose or hemicellulose constituting crystalline cellulose or gaps in cellulose fibers, resulting in expansion.
Next, a saccharified solution is obtained by saccharifying the pretreatment product using the saccharifying enzyme. Subsequently, a microbe added to the saccharified solution is allowed to grow so that the saccharified solution is fermented, thereby forming ethanol. Bioethanol is produced by distilling the obtained ethanol solution. In this respect, the problem is that low fermentation efficiency in the fermentation requires large energy for the distillation and increases cost.
To solve the problem, a technique of elevating a sugar concentration by mixing molasses of sugarcane with the saccharified solution, and fermenting the obtained mixed solution has been proposed (see Japanese Patent Laid-Open No. 2012-170443). According to the conventional technique, fermentation efficiency in the fermentation can be improved.
However, the problem of the conventional technique is also that the fermentation efficiency cannot be improved in some cases.
The present inventors have studied a reason why the fermentation efficiency may not be improved in the conventional technique. As a result, it has been found that the saccharified solution contains compounds, such as acetic acid, formic acid, and p-coumaric acid, formed as by-products during the pretreatment and the saccharification, and the growth of the microbe is inhibited by the compounds during the fermentation. In the present application, such a compound which inhibits the growth of the microbe is referred to as a “growth inhibitor”.
The growth inhibitor inhibits the growth of the microbe in the case where bioethanol is formed by fermenting the saccharified solution as well as in the case where useful materials such as biodiesel fuel or the like are obtained.
Thus, to reduce the influence of the growth inhibitor, a technique of removing the growth inhibitor from the mixed solution by filtering a mixed solution in which the saccharified solution is mixed with the molasses through a nanofiltration membrane or a reverse osmosis membrane has been proposed (see International Publication No. WO 2012/118171).
However, the filtration of the saccharified solution using the nanofiltration membrane or the reverse osmosis membrane requires a filtration step. This is inconvenient because production steps are complicated while a time required for bioethanol production is increased, and furthermore, a facility for filtration is necessary.